1. Field of the Invention
This invention generally relates to instrument calibration aids and more particularly pertains to devices and methods for checking the roundness of spherical contact surfaces. Description of the Prior Art
Spherical contact surfaces are utilized in a variety of instruments designed to precisely measure dimensions of length. Examples of such instruments include the various gages employed to measure critical dimensional parameters associated with a threaded member such as its pitch diameter, lead and taper. The use of spherical contact surfaces allows a threaded surface to be engaged within the threading itself as opposed to on its crests or in its troughs and thereby renders measurements less susceptible to error due to thread wear or contamination.
These spherical contact surfaces, commonly referred to as ball-point type contacts, are typically attached to a precision dial gage or caliper which is configured to provide a very precise and accurate measure of the absolute distance between two such contacts or perhaps between one such contact and a flat reference surface by either direct or comparative means. Despite the fact that such ball-point type contacts are typically manufactured of tungsten or tantalum carbide, they are subject to wear with use which causes the accuracy and precision of the resulting measurement to be compromised. In order to ensure that accurate and precise results are obtained, it is therefore imperative that the condition of the contact's spherical surface is checked on a regular basis.
When a spherical surface comes in contact with, for instance, a line-pipe type threading (60.degree. V-profile), only certain areas of the ball-point type contact are subject to wear. Simply indiscriminately measuring the ball's overall diameter may therefore not reveal wear as the ball's diameter is diminished only when measured in certain orientations. Consequently it is more accurate to say that the ball's "roundness" is decreased rather than its overall diameter. Moreover, a relatively small flaw in a ball-point type contact's "roundness" can have a greatly exaggerated effect on the accuracy of a measurement as a relatively small amount of surface wear allows the ball to be inserted to a much greater depth into a V-shaped groove. The roundness of the ball-point type contact is therefore of critical importance.
Two methods are commonly employed to check the roundness of the ball-point type contacts. One calls for the use of an apparatus which optically enhances the image of the sphere or ball-point so as to facilitate visual detection of any surface irregularities. The size and complexity of the instrument in conjunction with the fact that a subjective determination is ultimately required severely compromises the utility of this method. An alternative method involves the use of a micrometer or the equivalent with which a direct diameter measurement of the ball-point is taken. Multiple measurements taken by this method along various orientations of the ball-point increase the likelihood of detecting a worn area, but this method, at best, provides only a very rough and unreliable estimate of the amount of surface wear.
In view of the above described disadvantages associated with heretofore used methods of checking the roundness of the ball-point type sphere, a compact, simple, and economical means for quickly and accurately checking the roundness of a spherical surface is called for.